Forward osmosis treatment is a type of treatment in which solutions with different solute concentrations are contacted through a semi-permeable membrane, and the difference in osmotic pressure created by the difference in solute concentrations is used as the driving force to cause water to permeate through the semi-permeable membrane, thus causing migration of water from the dilute solution with low solute concentration to the concentrated solution with high solute concentration. Forward osmosis treatment allows concentration of dilute solutions, or dilution of concentrated solutions.
Forward osmosis treatment is similar to reverse osmosis treatment in that water is caused to permeate preferentially over solutes using a semi-permeable membrane. However, forward osmosis treatment utilizes the difference in osmotic pressure to cause migration of water from the dilute solution side to the concentrated solution side, and in this regard it differs from reverse osmosis treatment whereby pressure is applied to the concentrated solution side to cause migration of water against the difference in osmotic pressure, from the concentrated solution side to the dilute solution side. A semi-permeable membrane used for reverse osmosis treatment, therefore, is not necessarily suited for forward osmosis treatment if directly applied for forward osmosis treatment.
In reverse osmosis treatment, a concentrated solution is disposed on one side of a semi-permeable membrane while a dilute solution is disposed on the other side, and pressure at or greater than the difference in osmotic pressure of both solutions is applied to the concentrated solution to cause migration of water from the concentrated solution side to the dilute solution side. Therefore, a membrane used for reverse osmosis treatment (a reverse osmosis membrane) must have strength able to withstand the pressure at the concentrated solution side. In order to satisfy this requirement, it is necessary to ensure strength for the support layer that reinforces the thin membrane layer that exhibits semi-permeable membrane performance (also known as the skin layer or barrier material). The porosity of the support layer therefore cannot be increased to any very high extent. This consequently limits the space in which the solute in the support layer can freely diffuse. In reverse osmosis treatment, however, the direction of water permeation is the same as the leakage direction of the solute (salt), and therefore interior concentration polarization of the solute in the support layer does not take place. Consequently, the structure of the support layer has no definitive effect on the amount of water permeation through the membrane (also known as the membrane flux). In reverse osmosis treatment, therefore, as the pressure applied at the concentrated solution side increases, it is possible to also increase the amount of water permeating the semi-permeable membrane and migrating (the water permeation volume).
In forward osmosis treatment, on the other hand, the interior concentration polarization of the solute in the support layer has a major effect on the water permeation volume of the membrane. In forward osmosis treatment, a concentrated solution is situated on one side sandwiching the membrane (forward osmosis membrane), while a dilute solution is situated on the other side, and the difference in osmotic pressure between the two solutions is used as the driving force to cause migration of water from the dilute solution side to the concentrated solution side. In order to increase the water permeation volume of the forward osmosis membrane during this time, it is important to maximally reduce the interior concentration polarization of the solute in the support layer reinforcing the thin membrane layer which exhibits the semi-permeable membrane performance, to increase the effective osmotic pressure difference of the thin membrane layer. If the space in which the solute can freely diffuse in the support layer is limited, interior concentration polarization of the solute in the support layer will take place, making it impossible to ensure adequate water permeation volume. In a forward osmosis membrane, therefore, it is important to compose the support layer of a material that has a high enough porosity to avoid restricting interior diffusion of the solute as much as possible, and that is able to ensure the prescribed strength.
Various semi-permeable membranes have previously been investigated as forward osmosis membranes. For example, PTL 1 discloses a forward osmosis membrane having a thin membrane layer made of polyamide laminated on a support layer made of polyacrylonitrile, polyacrylonitrile-vinyl acetate copolymer, or polysulfone;
PTL 2 discloses a forward osmosis membrane having a thin membrane layer made of polyamide laminated on a support layer made of an epoxy resin; and
PTL 3 discloses a forward osmosis membrane having a barrier material coated on a support layer made of polyethylene terephthalate (PET) or polypropylene.
However, forward osmosis membranes with both high water permeability and high separation performance have not yet been obtained, and forward osmosis membranes with higher performance are desired.
Incidentally, forward osmosis treatment is usually carried out using a module comprising a forward osmosis membrane made into an appropriate form, packed into an appropriate container. Since a module using a forward osmosis membrane with a hollow fiber form can increase the fill factor of the membrane per module compared to a module using a forward osmosis membrane with a flat membrane form, it is considered more suitable in that it allows construction of a compact water purification system (NPL 1).
Macromolecular forward osmosis membranes using high molecular weight materials as the thin membrane layers are advantageous in terms of water permeability, and are therefore promising for application to forward osmosis membranes. However, the conventionally known macromolecular forward osmosis membranes are problematic in terms of durability against acids and organic solvents, as well as heat resistance, and are therefore limited in their scope of use.
In this regard, PTL 4 has proposed a forward osmosis membrane flow system that improves durability while maintaining the water permeability advantage of the macromolecular forward osmosis membranes, by using a forward osmosis membrane containing the inorganic material zeolite. However, although the forward osmosis membrane described in PTL 4 has improved durability, the water permeation volume is extremely low, and it has problems in terms of practicality as a forward osmosis membrane flow system.